1. Field of the Invention
The present invention relates to a measurement method which measures a target divided into a plurality of partial regions to measure a whole shape of the target by stitching these partial regions.
2. Description of the Related Art
Previously, in order to measure a shape of a target, there has been a method called stitching which measures the target divided into a plurality of partial regions having an overlapped region to stitch these partial regions. For example, as shown in FIG. 8, the target is measured by being divided into partial regions surrounded with four circles (1) to (4) indicated by dotted lines. The shape of the target is measured by stitching the divided and measured partial regions. However, there is a problem that an error is generated in performing the stitching.
The error in performing the stitching is roughly caused by two reasons. One is an error corresponding to a position change of the target when each partial region is measured (a setting error). The setting error has different value for each partial region, and corresponds to a lower-order error. The other is an error that is specific in a measurement device used when each partial region is measured (a system error). The system error has a common value for all partial regions, and corresponds to a relatively higher-order error.
FIGS. 9A and 9B are illustrations of a stitching error in an overlapped region of partial regions. For example, when the system error is not contained, as shown in FIG. 9A, overlapped regions of partial regions adjacent to each other have an identical shape. Therefore, the overlapped regions can be strictly coincident with each other if the position relation between the partial regions adjacent to each other. However, when the system error is contained, as shown in FIG. 9B, overlapped regions adjacent to each other do not have an identical shape. Therefore, even when the position relation between the partial regions adjacent to each other is aligned, these positions can not be strictly coincident with each other. As a result, a stitching error is generated.
As a method of correcting these errors, for example Japanese Patent No. 3162355 discloses a method of performing a coordinate conversion of measurement data of each partial region so that one of the measurement data of overlapped regions of two partial regions fits the other measurement data. Japanese Patent Laid-open No. 2001-66123 discloses a method of extracting only shape components of high frequency to perform a stitching so that two shape components of high frequency in overlapped regions are coincident with each other. U.S. Pat. No. 6,956,657 discloses a method of performing a correction so that a free compensator (corresponding to a setting error) and an interlocked compensator (corresponding to a system error) are entirely optimized. These error corrections enables highly accurate stitching.
However, in a conventional technology, a partial region that is a reference is determined and the stitching is performed so that the partial region fits another partial region. In other words, the setting error in the partial region that is a reference is fixed and the setting error of other partial regions is variable to align the reference partial region with other partial regions. Therefore, the calculation result contains the influences of the setting error of the reference partial region. Because the setting errors are different for partial regions and correspond to variability of the calculation results, there was a problem that it influences the measurement accuracy.